1. Field of the Invention
This invention relates to a method of forming high purity metal silicide targets such as WSiz, MoSiz, TiSiz, TaSi) used for sputtering, wherein 0,5&lt;z&lt;3.
2. Description of the Prior Art
Metal silicides have received attention for applications in VLSI circuits. These materials are attractive for gate metallization of Silicon based devices because their resistivities are an order of magnitude lower than those of conventional materials used for interconnection, such as alumina.
It is known from the article of C. G. SRIDHAR et al entitled "Characterization of Tungsten Silicide films deposited from ultrahigh density composite target", proceeding of the 1985 workshop "Tungsten and other refractory metals for VLSI applications"--Materials Research Society--p. 267-280--to obtain metal silicide films with a sputtering technique.
In order to improve the sputtering technique, the quality of the targets remains a key factor : a high purity target is required to obtain a pure silicide film on the substrate. Especially, it is known that the impurity levels of radioactive elements such as Uranium and Thorium which can emit Alpha-particles, and alkalis metals (Na,K) must be very low.
It is also known from the article of M. SUZUKI entitled "Development of refractory metal and silicide targets and their characteristics" Proceeding of the 1986 Workshop "Tungsten and other refractory metals for VLSI application II", 1986--Materials Research Society--p. 339-345, to make metal silicide targets (MSix) used for sputtering by mixing and sintering a metal (M) powder and a silicon powder.
As the purity is an essential property expected from these targets, a very complex and long chemical process is used in order to purify the raw materials:
At first, the raw materials are dissolved in acidic solution. Afterwards, crystallization and drying steps are carried out, the metal oxyde thus obtained being generally reduced by hydrogen in order to produce a metal powder, which is subsequently chemically refined, mixed with a Silicon powder, and then sintered at high temperature. Finally a machining step is often necessary to obtain the metal silicide target having an appropriate shape. Moreover, in addition to the complexity of this process, several other problems are encountered which means several drawbacks:
Since numerous steps are necessary to make said target, contamination by the surrounding environment cannot be avoided and the purity rarely overcomes more than four nines, i.e. 99,99% of metal purity based only on metal impurities. PA1 The sintering step of these refractory materials MSix is carried out at a temperature which is usually greater than 1500.degree. C. PA1 The cost of the overall process is high because there are several different steps which are time and/or energy consuming. PA1 the ratio of flowrates SiH.sub.4 /MF.sub.6 is greater than about 3. PA1 the temperature deposition is preferably set between about 200.degree. C. and 600.degree. C. PA1 high purity of the target of the invention compared to the targets obtained according to the teaching of Suzuki. PA1 one step process (continuous). PA1 low temperature process (less than 800.degree. C. instead of at least 1500.degree. C.). PA1 adjustable composition process: the stoechiometry of the formed silicide can be very precisely adjusted as a function of injected gas flow rates, including a modification of the flow rates during the process if the z number (MSiz) has to be modified.